This invention relates to a process for production of thiophosphonates. In particular the invention relates to a process for production of thiophosphonates which in general have the formula ##STR2## in which R is an alkyl, haloalkyl or aryl group; X is chloro or SR.sub.2 ; R.sub.1 is alkyl or aryl; and when X is SR.sub.2 then R.sub.2 is a group identical to R.sub.1, or R.sub.1 and R.sub.2 taken together form an optionally alkyl-substituted polymethylene group. In the last mentioned case, the compounds have a cyclical structure. Preferred alkyl or haloalkyl groups for the above substituents are those containing from 1 to 8 carbon atoms. Examples include methyl, ethyl, n-propyl, isopropyl, the various butyl, pentyl, hexyl, heptyl and octyl groups, and halogenated alkyl groups of such types, for instance halomethyl such as chloromethyl. When R.sub.1 and R.sub.2 are combined, the group will be a polymethylene chain having from 2 to 4 methylene groups, optionally substituted by one or more C.sub.1 -C.sub.3 alkyl groups. Examples of aryl groups are preferably phenyl and phenyl substituted by C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy, halo, nitro, phenyl and/or phenoxy.
A number of different processes have been described for production of such compounds. In one process a thiophosphonodichloride is reacted with a mercaptan in the presence of a substantial amount of a hydrogen chloride acceptor or base. This reaction can be generally described as ##STR3## when R.sub.1 (or R.sub.2) is an alkyl or aryl group or ##STR4## when R.sub.1 +R.sub.2 form a polymethylene group (R is as described above). As can be seen from these reaction schemes, the base is normally used in an amount of 2 equivalents per equivalent of the thiophosphonodichloride. It is generally either a tertiary amine or an inorganic base, for instance, sodium hydroxide.
One such process is described in U.S. Pat. No. 3,094,405 of Toy et al. The preferred base is a tertiary amine. The reference also discloses that the reaction may be carried out in the absence of a base, although no such examples are included.
European Patent Application No. 239,274 of Sumitomo Chemical Co. discloses several processes for production of di-(tertiary butyl)alkyl phosphonotrithioates, one of which involves reaction of an alkyl thiophosphonodichloride with t-butyl mercaptan in the presence of stoichiometric quantities of aqueous sodium hydroxide, together with a quaternary salt catalyst and/or a copper catalyst.
U.S. Pat. No. 4,352,762 of Fahmy discloses production of a phosphonodithioic monohalide from the corresponding dihalide by reaction of only one equivalent of mercaptan and one equivalent of base.
USSR Patent No. 186,464 discloses production of what are termed trithiophosphinic acid esters by reaction of a thiophosphinic acid dichloride with two equivalents of a mercaptan in the presence of an amine salt of a polythiophosphoric or polythiophosphinic acid as catalyst. USSR Patent No. 187,797 discloses production of a phosphonothioic monohalide from the dihalide and one equivalent of a mercaptan in the presence of such a catalyst.
Other processes for production of compounds of this type include the two-step process of reacting a phosphinic dichloride with a mercaptan, followed by oxidation or sulfuration of the product (for instance, U.S. Pat. No. 3,162,570 of Wilson) by the reaction of an alkylthiophosphonodichloride with an alkali metal mercaptide (U.S. Pat. No. 4,752,604 of Chavdarian et al.), or by the reaction of a dithiophosphonic anhydride with a mercaptan (USSR Patent No. 289,096).